how long do I allow my turbo engine care to idle before and after driving ?

First of all, so long as the car is on, the turbo is <i>always</i> spinning, it's just that at idle or easy driving, it's not spinning fast enough to make boost. So the issue isn't really turning the car off while the turbo is spinning (on or off boost). The main reason you'd want to idle the car after driving it is to cool the turbo down.
After hard use- extended periods "on boost" like racing- can raise the temp of the turbo to over 1000 degrees, so turning the engine off w/o letting the coolant and oil circulate through and cool it off can lead to cooking the bearings and oil.
In your case though, in town and rarely on boost, I wouldn't worry about it. I doubt the turbo gets anywhere near hot enough. If you really are concerned though, you can always invest in a "Turbo Timer" which keeps the engine running for a predetermined amount of time after you've left and locked the car.
That's up to you, but personally, I wouldn't bother. I've had a number of turbocharged cars that went 20+ yrs and never had a problem w/ the turbo cooking itself.